Gravimetric Standard Gas Mixtures for Global Monitoring of Atmospheric SF₆

In this study, standard gas mixtures of SF₆ in synthetic air were gravimetrically developed as a suite consisting of 6 mixtures with mole fractions of SF₆ ranging from 5 to 15 pmol/mol. For precision in weighing the gas fills, an automatic weighing system coupled with a high sensitivity mass balance was used and a gravimetry precision of 3 mg (2σ) was achieved. Impurity profiles of the raw gases were determined by various analyzers. In particular, sub pmol/mol levels of SF₆ in the matrix components (N₂, O₂, and Ar) were carefully measured, since the mole fraction of SF₆ in the final step can be significantly biased by this trace amount of SF₆ in the raw gases of the matrix components. Gravimetric dilution of SF₆ by purity-assessed N₂ was performed in 6 steps to achieve a mole fraction of 440 pmol/mol. In the final step, O₂ and Ar were added to mimic the atmospheric composition. Gravimetric fractions of SF₆ and the associated standard uncertainty in each step were computed according to the ISO 6142 and JCGM 100:2008, respectively, and validated experimentally. Eventually, the SF₆ fraction uncertainty of the standard gas mixtures combined by uncertainties of gravimetric preparation and verification measurements were found to be nominally 0.08% at a 95% confidence interval. A comparison with independent calibration standards from NOAA shows agreement within 0.49%, satisfying the extended WMO compatibility goal, 0.05 ppt.